Hitherto, general users have been utilizing an interconnection system of the type shown in FIG. 1 to interconnect a power system and a non-utility generation system such as a cogeneration system. That is, in the upper substation 60, the transformer 2 lowers the voltage of the power system 1. The power with its voltage thus lowered is supplied via the breaker 3 to the houses of general users. In the house of any general user, the power is supplied through a breaker 4 to a load 5.
On the other hand, in the non-utility generation system, the breaker 6 connects the output of the AC generator 7 and the power system 1. The power output from the AC generator 7 is controlled by regulating the magnetic-field winding 8 of the AC generator by means of an automatic voltage regulator (AVR) 9. The output frequency of the AC generator 7 is controlled as a speed governor 11 adjusts the power of the engine 10, which drives the AC generator 7.
A fault-detecting means is provided. A rectifier 12 detects the output current of the generator 7. An error-detecting circuit 13 detects an anomalous current on the basis of the relationship between the output current and output voltage of the generator 7. The circuit 13 supplies an error signal to a fault trip circuit 20, which opens the breaker 6.
A protective means is provided. A rectifier 14 is connected to the output of the breaker 6 (namely, at the side of the substation 60), and an over-current relay (OC) 19 drives the fault trip circuit 20. When the power system 1 has a trouble, particularly when it is blocked because, for example, the breaker 3 opens, the output power of the AC generator 7 is supplied to the load 5. Consequently, the frequency and the voltage come to have an anomalous value. This event is detected by an under-frequency (UF) relay 15, an over-frequency (OF) relay 16, an over-voltage (OV) relay 17, an under-voltage (UV) relay 18, and the like. The fault trip circuit 20 receives the detection signals from these relays and generates a trip command to the breaker 6 and opens the breaker 6. Hence, the load 5 can be protected, and the breaker 3 can be closed again. When a trouble occurs in the power system 1, opening the breaker 3, the output power of the AC generator 7 and the power required by the load 5 may have each an active component and an idle component which are equal to each other. If this case, both the frequency and the voltage change little, and none of the relays 15 to 19 operate. Therefore, the generator 7 continues to operate, causing a so-called isolated operation (islanding). As a consequence, an accident may take place, possibly preventing the breaker 3 from being closed again.
For the purpose of preventing such an isolated operation, a transfer breaker 61 is connected by a leased line to the substation 60, for performing transfer breaking on the breaker 6. The transfer breaker 61 supplies a break signal to the breaker 6, thereby opening the breaker 6, when it detects a signal which indicates that breaker 3 is opened in the upper substation 60.
The transfer breaker 61 needs to be used if the upper substation 60 is very far or if many houses receive power from the substation 60. If used in a non-utility generation system of medium capacity, whose output is only hundreds of kilowatts, the transfer breaker will increase the cost very much, scarcely resulting in a practical advantage in system interconnection.
If the upper substation 60 is very far or if many houses receive power from the upper substation 60, the transfer breaker 61 must be used, because it does not much increase the cost of the entire power system and does results in a practical advantage in system interconnection.
However, if the transfer breaker 61 is provided in a non-utility generation system of medium capacity whose output is only hundreds of kilowatts, the cost will much increase. In such a non-utility generation system, it scarcely achieves a practical advantage in system interconnection.
The object of the present invention is to provide an apparatus for protecting a non-utility generation system to be interconnected to a power system, which can reliably detects the isolated operation of the non-utility generation system connected to a power system, without using a transfer breaker which is an expensive device.